Slight judders when idle!

[Chrisc_91]

I've asked this question before but it's doing my head in now, I had my engine rebuilt with new Pistons a few weeks ago so I know it's not that, but even before I had the rebuild I was having a slight judder when idle when the revs settle down to say 750ish or just under, it's not all the time either, could this be the timing belt? I really don't know and this rocco is slowly getting itself in my bad books.

[Cuprabob]

Your car doesn't have a timing belt, it's a chain. I very much doubt it's anything to do with that.

[Chrisc_91]

Shows how much I know lol thanks for this info buddy

[blower]

Here has what's reduced my intermittent idle judders to near zero, in order of having the most effect:

-Full intake and valve clean (when i rebuilt my engine with forged pistons and took the head off)
-Setting plug gap to 0.8mm and using genuine latest plugs (03c905601B) or NGK 96209 - SIZFR6B8EG
-Using Sainsburys super 97 fuel....i find my idle is more stable on this, certainly better than Tesco momentum 99 which is rumored to have a lot of ethanol in it. Shell vpower is not bad, but again Sainsburys gives a better idle (maybe less ethanol)
-Audi R8 Coil packs

Now i still, rarely, can feel a bit of an idle judder if i sit at idle for a long time - but its rare, vast majority of time i feel nothing when waiting at the lights or just let it idle when parked etc.
I do get a bit of an idle judder if the engine has not fully warmed up, especially when it starts to read between 50c and 90c on the gauge - if its in that range and i stop at the lights it can sometimes be a bit juddery....once warmed up it rarely judders at idle now.

All these 160ps engines are juddery at idle, infact all TSI/FSI engines can be juddery at idle - just some more than others, and a lot of this is down to the compromise on spark plugs.
The new mk7 golf 1.4 TSI, a completely different engine suffers from it, so does new the GTI, the old GTI etc - its just taken as normal....just the 1.4 does it more frequently than others so its noticed more.
I've done a post on this as to the reasons why, on another forum, i'll post it here.

A note on plugs:
There is a lot of misinformation around what plugs to use for this engine...

Firstly don't use BKR7EIX, these plugs are shorter, tip sits higher up in the cylinder which reduces ignition efficiency at lower loads (idle). They should only possibly be used for mapped engines, and even then you have to accept the idle is going to be noticeably worse.
Secondly, contrary to a lot of information out there - a smaller gap will always result in a poorer idle. The trick is to have the gap as big as you can without resulting in misfires under full boost - this will give you the best idle.
Stock plugs are rated to be 0.7-0.8mm....in reality, when new they are somewhere between 0.7 and 0.75mm - opening them up to 0.8mm improves idle, you can go as far as 0.85mm but no more than that as you may start to introduce misfiring at high boost. If you open the gap up you should check them every 10k miles to ensure the gap has not increased, and regap if necessary.

[blower]

This idle problem is common on all TSI/TFSI/FSI engines, and even the new ones have it, part of the issue is the never ending chase for lower emissions and better fuel economy. The TSI's run at a pretty low idle which is going to exacerbate the primary cause of what my research and testing has led me to conclude....

Firstly these 'twitches' are not full on misfires, they are an incomplete burn, a poor ignition of the air fuel mixture. At minimum load, lowest airflow, the air fuel mixture is not very well mixed - this is particularly the case in direct injection engines.

The primary issue lies with the spark, or more precisely the flame kernel resulting from the spark ignition.....In highly boosted engines, with a lot of HP per cylinder - you need to have small spark plug gaps so that you don't get misfires under full boost. However the smaller the gap the weaker the initial flame kernel that results from ignition.

With a spark plug you always want the biggest gap you can get away with without misfiring, under all conditions. VW have to have 0.7-0.8mm gaps on high boost engines - the problem with this is this small gap results in a weaker flame kernel...if the fuel is not perfectly mixed and conditions not just right, it can result in an incomplete or delayed burn - causing the engine twitches you feel at idle. Idle being the worst case scenario, with conditions being right on the limit.

With the low idle rpm, each ignition is running on the edge of an acceptable burn efficiency....it only takes some slight variations in the input variables (fuel quality, air quality (humidity), some intake carbon buildup etc) to tip the balance and result in some poor burns during the spark ignitions.

I have shown this to be the case with my own engine - if i open the spark plug gap up, the twitching stops completely - but then i can't run full boost without running the risk of misfires, which at full load is when you really can't afford them.

It's also why, for the older series of 1.4 TSI engines - the CAXA varient thats 120hp has a much better idle than the 160hp twincharger - the CAXA engine is the same block, same injectors, HPFP etc but the key difference is it uses 0.9-1.0mm spark plugs...fine for the lower boost levels of that engine, but no good for the high boost of the twincharger.

Also added into the mix is the spark plug heat range: Pushing a lot of HP / boost out per cylinder brings another problem with it - at higher power output and hence cylinder temperature, you will need a colder plug to stop it melting, but at lower power you need a hotter plug to ensure carbon deposits are burn't off and you get a good spark ignition. As you increase the HP per cylinder, your widening the gap between low and high power...add in improvements in efficiency where by cylinder temperatures are lower when cruising - and you end up having to severely compromise what heat range plug you should have....you have to have a plug cold enough to cope with a heavy maximum load / boost condition. The issue is, that at low loads carbon is not being burn't off fully on the tips of the spark plugs, as the plug is too cold a heat range - this results in a poorer spark and adds another negative variable into the idle burn efficiency mix.

This idle issue is prevalent in all high boost TSI/FSI engines. It's also not just VW engines...any direct injection engine that runs high boost levels per cylinder and low idle rpm is in a similar boat. Its worse in some engine variants than others, which is purely down to how well the air and fuel are mixed, and subsequently ignited - this is influenced by injector position, spray pattern and also spark plug positioning within the cylinder.....however they all seem to be operating on the edge of acceptable burn, and it doesn't take much to get them twitching at idle.

[Chrisc_91]

Thanks for the info once again mate much appreciated!

[janherko]

You really have some reasonable inputs blower, but can you please tell me if it´s possible to raise idle speed on our engine? Just like on 2.0 TFSi Audis or TDis I heard of...I´m sure it could help a lot.
VCDS has an option in Engine Controller called Idle Speed Adjustment, mine requires Security Code, which I´m yet to obtain...